Up-converters generally take lower frequency signals and convert them to higher frequencies. Often, these up-converted signals need to have their power varied, depending upon their initial power and frequency. A variable gain function generally produces the needed power change.
Implementation of a variable gain function at microwave frequencies can present some issues. At microwave frequencies, such as in the range of 6 to 24 GHz, the two octave frequency span is large enough such that traditional narrowband techniques cannot easily be used. The resulting design therefore must provide a broadband range.
At microwave frequencies, designs need to minimize parasitic elements, in particular capacitance. Typically, one would implement an attenuator in a ladder fashion. However, this type of structure has too much capacitance because of the repetitive resistor network.
U.S. Pat. No. 4,837,530 illustrates one approach that uses a voltage variable attenuator (VVA). At a minimum this requires buffering at the output to achieve any gain. In addition, the outputs of these circuits are normally controlled to be linear-in-volts. Linear-in-volts controls make the use of adjustable gain controls difficult without some added translation to linear-in-dB.
Another approach, discussed in U.S. Pat. No. 7,382,190, uses a two-stage interpolator. For example, the interpolator shown in FIG. 5 drives what is called the signal steering core. The signal steering core together with the current mirror and resistors RC in FIG. 5 of the Gilbert patent comprise the second stage of the interpolator. From a noise and impedance standpoint, using a 2-stage interpolator may be less than ideal at microwave frequencies.